CODEVS 3023 魔法禁书目录1：追捕大妈

#include<iostream>
#include<cstdio>
#include<cstring>
#define maxv 20050
#define maxe 40050
using namespace std;
struct edge
{
long long u,v,w,nxt;
}e[maxe];
long long n,a,b,c,g[maxv],size[maxv],nume=0,fath[maxv];
void addedge(long long u,long long v,long long w)
{
e[++nume].u=u;
e[nume].v=v;
e[nume].w=w;
e[nume].nxt=g[u];
g[u]=nume;
}
void dfs(long long x,long long father)
{
fath[x]=father;size[x]=1;
for (long long i=g[x];i;i=e[i].nxt)
{
long long v=e[i].v;
if (v!=father)
{
dfs(v,x);
size[x]+=size[v];
}
}
}
int main()
{
scanf("%lld",&n);
for (long long i=1;i<=n-1;i++)
{
scanf("%lld%lld%lld",&a,&b,&c);
addedge(a,b,c);
addedge(b,a,c);
}
dfs(1,0);
long long ans=0;
for (long long i=1;i<=nume;i+=2)
{
a=e[i].u,b=e[i].v,c=e[i].w;
if (fath[a]==b) swap(a,b);
ans+=c*size[b]*(n-size[b]);
}
double now=(double)ans/(n*n-n);
printf("%.2lf\n",now*2);
return 0;
}